G.994.1 provides a flexible mechanism for DSL transceivers to exchange capabilities and select a common mode of operation. However it lacks the capability to prevent two or more transceivers from training up against one remote transceiver in high cross-talk environments, where one connection is via a crosstalk channel.
For example, quite often, while one transceiver trains up against the remote transceiver on the direct channel, another transceiver trains up against the same remote transceiver on the FEXT channel. This FEXT channel transceiver quite often passes handshake and several phases of channel discovery before failing activation. This false training can be quite problematic for vectored VDSL2 (G.vector) systems, and could cause other lines to retrain as well.
In vectored VDSL2, for example, it has been observed that after a handshake session one VTU-O (i.e. central office (CO) based VDSL2 modem) may establish connection with two VTU-R's (i.e. customer premises location based modems (CPE)) simultaneously. In this event, as an example, it is possible that two modems on the CPE side of the line may try to initialize with a single modem on the CO side of this line. Under normal operating conditions, specifically VDSL operating without vectoring, the CPE modem on the direct channel will (hopefully) observe significantly better SNR during the training phase of initialization and that across the crosstalk channel will observe a very poor SNR and eventually abort initialization. However, if this event were to occur in a Vectored VDSL2 system, then some issues can occur in the updating of the channel matrix when a new line is joining and a secondary line via the crosstalk path are both trying to join the vectored group; this is the situation demonstrated in FIG. 1.
As shown in FIG. 1, there are a number of lines (represented by VTU-O#1 thru VTU-O#3 in a CO-side DSLAM 110) operating in a vectored group 102 and in Showtime; the crosstalk within the vectored group is being cancelled by the VCE in the CO. As further shown, Line #J1 associated with CPE modem VTU-R #J1 106 desires to join the vectored group 102. During handshake, however, there are two CPE modems communicating with VTU-O#J1, the desired VTU-R#J1 106 and another modem VTU-R#J1F 104 via the crosstalk coupling path. During Handshake, it is possible that both CPE devices establish communication with VTU-O#J1 and enter the initialization phase. During the channel discovery phase, pilot sequences are communicated in the downstream direction via the O-SIGNATURE message and if upstream vectoring is enabled, then pilot sequences for the upstream channel will be communicated to the CPE for application in the upstream direction in training sequence R-P-Vector 1. In this case two CPE modems will be sending the same pilot sequence upstream in R-P-Vector 1 during Channel Discovery. The fact that two modems are sending the same pilot sequence can cause confusion in updating the crosstalk channel matrix in the upstream direction. It is not until the training phase where SNR measurements are made and one line can be dropped due to poor SNR readings but by this time some potential harm may have been done to the upstream crosstalk channel matrix.
It is strongly desired that connection out of handshake be done with only one modem pair and avoid application of one pilot sequence across more than one line, even if the connection were established via the crosstalk channel as opposed to the direct channel.